A hybrid is an analog electrical network that is part of the Analog Front End (AFE) of a modem (e.g., Digital Subscriber Line (DSL) modem, etc.) and is responsible for the two to four wire conversion, allowing for real-time, full-duplex transmission of 15 signals. A hybrid allows a transmit signal from one pair of the 4-wire side (transmit) to pass directly on to the 2-wire side (subscriber loop) and at the same time suppresses the transmit signal component on the other pair of the 4-wire side (receive). A hybrid is basically an electrical bridge, and can come in different designs such as single-ended and differential hybrids. In FIG. 1 there is shown a block diagram of a typical prior art modem AFE 100.
Ideally, a hybrid should completely suppress the transmit energy on the receive wire pair. However, in reality, since the line impedance changes from one subscriber loop to the next, the hybrid can only be designed to match an “average” loop's impedance. In many cases, with bridge taps, home phone network adapters and other electrical devices attached to the telephone line, loop impedances can differ substantially from this predetermined “average” or nominal value. In cases where the subscriber loop has an impedance different from this predetermined average, the hybrid is mismatched to the line, and the transmit energy can completely dominate the receive signal and therefore the receive performance suffers.
In order to reduce the effect of hybrid mismatch on the modem receive performance, multiple hybrids are often added to a modem in order to allow for selection of a hybrid that will provide the best performance. With multiple hybrid designs, the important task becomes the selection of a hybrid from among the plurality of available hybrids in order to maximize the receive performance.
A typical prior art method for selecting the optimal hybrid is to train up the modem with each hybrid and then choose the hybrid that achieves the maximum data rate. However, this would mean that for each loop the modem needs to be retrained as many times as the number of hybrids before the best hybrid can be selected. This is often unacceptable, as it would dramatically increase the training time for the modem.
As pointed out earlier, the goal of a hybrid is to minimize the transmit energy that gets coupled into the receive path. The transmit signal is the dominant portion of the transmit energy. Hence, other prior art techniques for dynamically choosing the best hybrid during modem training attempt to minimize the total echo energy due to the transmit signal in the receive path. This can be done by switching in the different hybrids during an appropriate segment of modem initialization, measuring the corresponding total echo energies, and then choosing the hybrid that gives the minimum energy. Unfortunately, this approach is not guaranteed to give the optimal hybrid in terms of maximum data rate performance. A need exists in the art for a method for dynamically switching a hybrid in a modem during modem initialization in order to maximize channel capacity and hence maximize performance.